1. FIELD OF THE INVENTION
The present invention relates to a cable-sheath material comprising an aluminum-base alloy having a characteristic of less eddy current loss compared with a conventional aluminum sheath, and superior workability to result a power transmission line having high current-carrying capacity.
2. DESCRIPTION OF THE PRIOR ART
Hitherto, lead, lead alloys, and commercial grade pure aluminum have been used as cable-sheath materials. That is, as well known, lead and lead alloys are excellent in corrosion resistance and workability and provide less eddy current loss due to their low electrical conductivity and hence they have long been used as indispensable cable-sheath materials. However, since lead and lead alloys are inferior in mechanical properties such as tensile stength, creep strength, and fatigue and are large in strain and deformation of the sheath by inside pressure, it is required to increase the thickness of the sheath and increase the offset of the cable during construction. On the other hand, pure aluminum is lower in workability and corrosion resistance as compared with lead and lead alloys and also provides large eddy current loss.
Recently with the increase of the demand for electric power, cables having large current-carrying capacity have been used and for such cable, sheath materials providing particularly lower eddy current loss has been required. For increasing the current-carrying capacity of a transmission cable, it may be considered to increase the cross section of conductor or increase the current density. However, in the case of increasing the cross section of conductor, such troubles occur that already installed sheaths and underground pipes are too small to use for the purpose. Also, in the case of increasing the current density by increasing the electric current with an already installed transmission line having the definite cross section, the eddy current loss and the Joule's heat effect become larger and thus in such cases there is a limitation in the increase of current-carrying capacity. In order to reduce the eddy current loss, it is required to reduce the electrical conductivity of sheath materials as low as possible. That is, since the eddy current becomes lower and the eddy current loss is reduced as the electrical conductivity of sheath materials decreases, thus the current-carrying capacity of a power transmission line shielded such sheath materials can be increased.
A conventional aluminum sheath has an electrical conductivity of 59% IACS (International Annealed Copper Standard) but if, for example, an aluminum alloy having an electrical conductivity of 40% IACS or 35% IACS may be used as a cable-sheath material, the eddy current loss at power transmission is reduced and thus it becomes possible to transmit an electric power of the same capacity as in a cable having a larger size by the already installed cable having a definite cross section (e.g., 1600 mm.sup.2).
An object of this invention is, therefore, to provide a cable-sheath material having a conductivity of lower than 55% IACS, preferably lower than 47% IACS, providing less eddy current loss, and having excellent workability required for making a cable sheath, high mechanical strength, proper ductility, and small specific gravity.
Now, the electrical conductivity of aluminum is reduced by the addition of alloying elements but if the content of the alloying elements becomes higher than solid solubility in aluminum, the effect of reducing the conductivity becomes less. The effect of reducing conductivity per unit weight is quite high in Cr, Mn, and Li, and then Ti and Zr follow, and the effect by Cu and Fe is comparatively small.
On the other hand, the workability of aluminum is also reduced by the addition of alloying elements. An aluminum sheath is generally fabricated by an extrusion method and the extrudability is evaluated by the extrusion pressure, extrusion speed, and the presence of defects in the sheath extruded. That is, the extrudability is evaluated to be excellent as the extrusion pressure is lower, the extrusion speed is higher, and the defects in the extruded sheath, such as striking and cracking are less.
As a conventional aluminum sheath, 99.7% aluminum or 99.5% aluminum which does not contain specific elements except impurities such as Fe and Si is used and the 99.7% aluminum is lower in tensile strength and proof stress at a high temperature than 99.5% aluminum but the elongation at a high temperature is same in the both aluminums. In the case of forming a sheath by extruding the cast block of such a metal, the extrusion pressure is lower and the extrusion speed is higher in 99.7% aluminum than in 99.5% aluminum, that is, 99.7% aluminum is quite excellent in workability as compared with 99.5% aluminum. Therefore, in order to reduce the eddy current loss without reducing the workability of the sheath material, it is desired to reduce the electrical conductivity of a sheath material without increasing the high-temperature tensile strength and proof stress and without reducing elongation property.